“... the optimist sees the rose and not its thorns; the pessimist stares at the thorns, oblivious to the rose,” Kahlil Gibran.

Optimists enjoy better health, more success, more happiness, and longer lives, than pessimists. No surprise, then, that psychologists are taking an increasing interest in our outlook on life. An unresolved issue is whether optimism and pessimism are two ends of the same spectrum, or if they’re separate. If the traits are separate, then in principle, some people could be highly optimistic and pessimistic – to borrow the poet Gibran’s analogy, they would be keenly aware of both the rose and its thorns.

Timothy Bates at the University of Edinburgh has turned to behavioural genetics to help settle this question. He’s analysed data on optimism and pessimism gathered from hundreds of pairs of identical and non-identical twins. These were participants from a US survey and their average age was 54. The twins rated their agreement with various statements as a way to reveal their optimism and pessimism such as “In uncertain times, I usually expect the best” and “I rarely count on good things happening to me.” They also completed a measure of the “Big Five” personality traits: extraversion, neuroticism etc.

The reasoning behind twin studies like this is that if optimism and pessimism are highly heritable (i.e. influenced by inherited genetic factors), then these traits should correlate more highly between pairs of identical twins, who share all their genes, than between non-identical twins, who share approximately half their genes. And if optimism was found to be more heritable than pessimism, or vice versa, this would indicate different genetic influences on optimism and pessimism.

Another insight from twin studies is to disentangle the relative influence of shared and unique environmental factors – these are the aspects of a twin’s upbringing that they share with their sibling, such as parenting style, and those that are unique, such as the friends they keep.

Bates’ analysis indicates that optimism and pessimism are subject to shared genetic influences (with each other, and with other personality traits), but also to independent genetic influences, thus supporting the notion that optimism and pessimism are distinct traits, not simply two sides of the same coin.

“Optimism and pessimism are at least partially biologically distinct, resulting in two distinct psychological tendencies,” Bates said. He added that this dovetails with neuroscience evidence that’s indicated there are separate neural systems underlying optimism and pessimism.

The new findings also suggested there is a “substantial” influence of upbringing on optimism and pessimism (i.e. increasing one and lowering the other, and/or vice versa). This raises the intriguing possibility that optimism might to be some extent a malleable trait that can be encouraged through a child’s upbringing.